🛰️ OAuth2::MCP
if ci_badges.map(&:color).detect { it != "green"} ☝️ let me know on Discord or RubyForum, as I may have missed the notification.
if ci_badges.map(&:color).all? { it == "green"} 👇️ send money so I can do more of this. FLOSS maintenance is now my full-time job.
I've summarized my thoughts in this blog post.
🌻 Synopsis
oauth2-mcp is a Ruby resource-server toolkit for securing HTTP Model Context
Protocol servers with OAuth 2.1-style bearer authorization.
It builds on the oauth2 gem for the Ruby OAuth/OIDC substrate and adds the MCP
resource-server pieces that generic OAuth clients do not provide.
The gem is intentionally focused on the MCP server side:
- protected-resource metadata for MCP endpoint discovery;
WWW-Authenticatebearer challenges;- scope and capability mapping for application policy layers;
- provider adapters for WorkOS/AuthKit and generic OIDC/JWKS validation;
- explicit audience/resource validation so tokens cannot be replayed against another MCP server.
Token passthrough is not a supported pattern. MCP servers should validate the incoming token for their own resource and issue separate downstream credentials when calling other services. Successful authorization results expose normalized claims and mapped capabilities, not the raw bearer token.
💡 Info you can shake a stick at
Compatibility
Compatible with MRI Ruby 3.2.0+, JRuby, and TruffleRuby.
CI workflows and Appraisals are generated for MRI Ruby 3.2.0+.
This test floor is configured by ruby.test_minimum in .kettle-jem.yml and
may be higher than the gem's runtime compatibility floor when legacy Rubies are
not practical for the current toolchain.
The amazing test matrix is powered by the kettle-dev stack.
Runtime and Platform Support
Support tiers describe what this project's CI actually exercises, not what may work in an untested environment:
- Parallel support: CI runs the test suite with multiple workers available.
- Unsupported: the engine/platform combination is not covered by CI and is not a project compatibility claim.
Federated DVCS
Enterprise Support
Available as part of the Tidelift Subscription.
The maintainers of this and thousands of other packages are working with Tidelift to deliver commercial support and maintenance for the open source packages you use to build your applications. Save time, reduce risk, and improve code health, while paying the maintainers of the exact packages you use.
- 💡Subscribe for support guarantees covering all your FLOSS dependencies
- 💡Tidelift is part of Sonar
- 💡Tidelift pays maintainers to maintain the software you depend on!📊
@Pointy Haired Boss: An enterprise support subscription is "never gonna let you down", and supports open source maintainers
Alternatively:
✨ Installation
Install the gem and add to the application's Gemfile by executing:
bundle add oauth2-mcp
If bundler is not being used to manage dependencies, install the gem by executing:
gem install oauth2-mcp
⚙️ Configuration
oauth2-mcp does not use global configuration. Build the objects that match the
OAuth provider and MCP endpoint you are protecting, then pass those objects to a
resource server or Rack middleware.
The core resource metadata is the MCP resource URI, the authorization server issuer(s), and the scopes your endpoint understands:
metadata = OAuth2::MCP::ProtectedResourceMetadata.new(
resource: "https://brain.example.com/mcp",
authorization_servers: ["https://example.authkit.app"],
scopes_supported: %w[memory.read memory.write]
)
Token validation is supplied by one of the validator adapters:
OAuth2::MCP::JWTValidatorfor JWT bearer tokens when you already have JWKS.OAuth2::MCP::OIDCDiscoverywhen the authorization server exposes OIDC discovery and JWKS metadata.OAuth2::MCP::WorkOSAuthKitfor WorkOS AuthKit resource-server JWTs.OAuth2::MCP::IntrospectionValidatorfor opaque tokens validated through an OAuth token introspection endpoint.
Map provider scopes to application capabilities with ScopeMapper when the MCP
server needs internal permission names:
scope_mapper = OAuth2::MCP::ScopeMapper.new(
mapping: {
"memory.read" => "documents_read",
"memory.write" => "documents_write"
}
)
Finally, configure the resource server with the metadata URL you publish to
clients. That URL is emitted in WWW-Authenticate challenges so clients can
discover how to obtain an acceptable token.
resource_server = OAuth2::MCP::ResourceServer.new(
resource_metadata: metadata,
resource_metadata_url: "https://brain.example.com/.well-known/oauth-protected-resource/mcp",
validator: validator,
scope_mapper: scope_mapper
)
🔧 Basic Usage
Build protected-resource metadata for an MCP endpoint:
metadata = OAuth2::MCP::ProtectedResourceMetadata.new(
resource: "https://brain.example.com/mcp",
authorization_servers: ["https://example.authkit.app"],
scopes_supported: %w[memory.read memory.write]
)
metadata.to_json
Return a bearer challenge when authorization is missing or insufficient:
challenge = OAuth2::MCP::BearerChallenge.new(
resource_metadata: "https://brain.example.com/.well-known/oauth-protected-resource/mcp",
scope: %w[memory.read],
error: "insufficient_scope",
error_description: "memory.read is required"
)
headers["WWW-Authenticate"] = challenge.to_header
Authorize an MCP HTTP request with a provider-specific token validator:
metadata = OAuth2::MCP::ProtectedResourceMetadata.new(
resource: "https://brain.example.com/mcp",
authorization_servers: ["https://example.authkit.app"],
scopes_supported: %w[memory.read memory.write]
)
validator = OAuth2::MCP::JWTValidator.new(
jwks: {"keys" => provider_jwks},
issuer: "https://example.authkit.app",
audience: "https://brain.example.com/mcp"
)
scope_mapper = OAuth2::MCP::ScopeMapper.new(
mapping: {
"memory.read" => "documents_read",
"memory.write" => "documents_write"
}
)
resource_server = OAuth2::MCP::ResourceServer.new(
resource_metadata: metadata,
resource_metadata_url: "https://brain.example.com/.well-known/oauth-protected-resource/mcp",
validator: validator,
scope_mapper: scope_mapper
)
result = resource_server.authorize(
request: rack_env,
scopes: ["memory.read"]
)
halt result.status, result.headers, "" unless result.allowed?
Protect a Rack-compatible MCP HTTP endpoint:
use OAuth2::MCP::RackMiddleware,
resource_server: resource_server,
scopes: ["memory.read"]
Or build the validator from OIDC discovery:
validator = OAuth2::MCP::OIDCDiscovery.new(
issuer: "https://example.authkit.app"
).jwt_validator(
audience: "https://brain.example.com/mcp"
)
For WorkOS AuthKit, use the provider adapter. It validates against the AuthKit
issuer and /oauth2/jwks endpoint:
validator = OAuth2::MCP::WorkOSAuthKit.new(
subdomain: "acme",
audience: "https://brain.example.com/mcp"
)
For opaque access tokens, use OAuth token introspection with an OAuth2::Client
configured for the authorization server:
validator = OAuth2::MCP::IntrospectionValidator.new(
client: oauth_client,
introspection_url: "https://auth.example.com/oauth2/introspection",
audience: "https://brain.example.com/mcp",
issuer: "https://auth.example.com"
)
🦷 FLOSS Funding
While ruby-oauth tools are free software and will always be, the project would benefit immensely from some funding. Raising a monthly budget of... "dollars" would make the project more sustainable.
We welcome both individual and corporate sponsors! We also offer a wide array of funding channels to account for your preferences. Currently, Open Collective is our preferred funding platform.
If you're working in a company that's making significant use of ruby-oauth tools we'd appreciate it if you suggest to your company to become a ruby-oauth sponsor.
You can support the development of ruby-oauth tools via GitHub Sponsors, Liberapay, PayPal, Open Collective and Tidelift.
Open Collective for Individuals
Support us with a monthly donation and help us continue our activities. [Become a backer]
NOTE: kettle-readme-backers updates this list every day, automatically.
No backers yet. Be the first!
Open Collective for Organizations
Become a sponsor and get your logo on our README on GitHub with a link to your site. [Become a sponsor]
NOTE: kettle-readme-backers updates this list every day, automatically.
No sponsors yet. Be the first!
Open Collective for Donors
Bill Woika Philipp Ebneter Grigoriy Guest
Another way to support open-source
I’m driven by a passion to foster a thriving open-source community – a space where people can tackle complex problems, no matter how small. Revitalizing libraries that have fallen into disrepair, and building new libraries focused on solving real-world challenges, are my passions. I was recently affected by layoffs, and the tech jobs market is unwelcoming. I’m reaching out here because your support would significantly aid my efforts to provide for my family, and my farm (11 🐔 chickens, 2 🐶 dogs, 3 🐰 rabbits, 8 🐈 cats).
If you work at a company that uses my work, please encourage them to support me as a corporate sponsor. My work on gems you use might show up in bundle fund.
I’m developing a new library, floss_funding, designed to empower open-source developers like myself to get paid for the work we do, in a sustainable way. Please give it a look.
Floss-Funding.dev: 👉️ No network calls. 👉️ No tracking. 👉️ No oversight. 👉️ Minimal crypto hashing. 💡 Easily disabled nags
🔐 Security
See SECURITY.md.
🤝 Contributing
If you need some ideas of where to help, you could work on adding more code coverage, check issues or PRs, or use the gem and think about how it could be better.
We so if you make changes, remember to update it.
See CONTRIBUTING.md for more detailed instructions.
🚀 Release Instructions
See CONTRIBUTING.md.
Code Coverage
🪇 Code of Conduct
Everyone interacting with this project's codebases, issue trackers, chat rooms and mailing lists agrees to follow the .
🌈 Contributors
Made with contributors-img.
Also see GitLab Contributors: https://gitlab.com/ruby-oauth/oauth2-mcp/-/graphs/main
📌 Versioning
This library follows for its public API where practical. For most applications, prefer the Pessimistic Version Constraint with two digits of precision.
For example:
spec.add_dependency("oauth2-mcp", "~> 0.0")
Dropping support for a platform can be a breaking change for affected users. If a release changes supported platforms, it should be called out clearly in the changelog and versioned with that impact in mind.
To get a better understanding of how SemVer is intended to work over a project's lifetime, read this article from the creator of SemVer:
See CHANGELOG.md for a list of releases.
📄 License
The gem is available as open source under the terms of the MIT .
© Copyright
See LICENSE.md for the official copyright notice.
- Copyright (c) 2026 Peter H. Boling
🤑 A request for help
Maintainers have teeth and need to pay their dentists. After getting laid off in an RIF in March, and encountering difficulty finding a new one, I began spending most of my time building open source tools. I'm hoping to be able to pay for my kids' health insurance this month, so if you value the work I am doing, I need your support. Please consider sponsoring me or the project.
To join the community or get help, use the RubyForum or Discord.
To say "thanks!" ☝️ Join the community or 👇️ send money.
Please give the project a star ⭐ ♥.
Many parts of this project are actively managed by a kettle-jem smart template utilizing StructuredMerge.org merge contracts.
Thanks for RTFM. ☺️
